Resonant Fabry-Perot semiconductor saturable absorbers and two photon absorption power limiters

ABSTRACT

An intracavity resonant Fabry-Perot saturable absorber (R-FPSA) induces modelocking in a laser such as a fiber laser. An optical limiter such as a two photon absorber (TPA) can be used in conjunction with the R-FPSA, so that Q-switching is inhibited, resulting in laser output that is cw modelocked. By using both an R-FPSA and a TPA, the Q-switched modelocked behavior of a fiber laser is observed to evolve into cw modelocking.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/264,804 titled “Resonant Fabry-Perot Semiconductor SaturableAbsorbers And Two Photon Absorption Power Limiters” filed Nov. 4, 2008,which is a divisional of U.S. patent application Ser. No. 11/197,852titled “Resonant Fabry-Perot Semiconductor Saturable Absorbers And TwoPhoton Absorption Power Limiters” filed Aug. 5, 2005, now U.S. Pat. No.7,453,913, issued Nov. 18, 2008, which is a continuation of U.S.application Ser. No. 09/738,372, filed Dec. 15, 2000, now U.S. Pat. No.6,956,887, issued Oct. 18, 2005, which is a divisional of U.S.application Ser. No. 09/149,368, filed Sep. 8, 1998, now U.S. Pat. No.6,252,892, issued Jun. 26, 2001. Each of the foregoing applications ishereby incorporated by reference in its entirety and made part of thisspecification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to modelocking, and inparticular, to cw modelocking in which Q-switched pulses are suppressed.

2. Description of the Related Art

Semiconductor saturable absorbers have recently found application in thefield of passively modelocked, ultrashort pulse lasers. These devicesare attractive since they are compact, inexpensive, and can be tailoredto a wide range of laser wavelengths and pulsewidths. Semiconductorsaturable absorbers were first used to passively modelock a diode laser(see P. W. Smith, Y. Silberberg and D. B. A. Miller, “Mode locking ofsemiconductor diode lasers using saturable excitonic nonlinearities,” J.Opt. Soc. Am. B, vol. 2, pp. 1228-1236, 1985 and U.S. Pat. No. 4,435,809to Tsang et al). Quantum well and bulk semiconductor saturable absorbershave also been used to modelock color center (M. N. Islam, E. R.Sunderman, C. E. Soccolich, I. Bar-Joseph, N. Sauer, and T. Y. Chang,“Color center lasers passively mode-locked by quantum wells”, IEEE J. ofQuantum Electronics, vol. 25, pp. 2454-2462 (1989)) and fiber lasers(U.S. Pat. No. 5,436,925 to Lin et al.).

A saturable absorber has an intensity-dependent loss l. The single passloss of a signal of intensity I through a saturable absorber ofthickness d may be expressed asl=1−exp(−αd)  (1)in which α is the intensity dependent absorption coefficient given by:α(I)=α₀/(1+I/I _(SAT))  (2)Here α₀ is the small signal absorption coefficient, which depends uponthe material in question. I_(SAT) is the saturation intensity, which isinversely proportional to the lifetime (τ_(A)) of the absorbing specieswithin the saturable absorber. Thus, saturable absorbers exhibit lessloss at higher intensity.

Because the loss of a saturable absorber is intensity dependent, thepulse width of the laser pulses is shortened as they pass through thesaturable absorber. How rapidly the pulse width of the laser pulses isshortened is proportional to |dq₀/dI|, in which q₀ is the nonlinearloss:q ₀ =l(I)−l(I=0)  (3)l(I=0) is a constant (=1−exp(−α₀d)) and is known as the insertion loss.As defined herein, the nonlinear loss q₀ of a saturable absorberdecreases (becomes more negative) with increasing intensity I. |dq₀/dI|stays essentially constant until I approaches I_(SAT), becomingessentially zero in the bleaching regime, i.e., when I>>I_(SAT).

For a saturable absorber to function satisfactorily as a modelockingelement, it should have a lifetime (i.e., the lifetime of the upperstate of the absorbing species), insertion loss l(I=0), and nonlinearloss q₀ appropriate to the laser. Ideally, the insertion loss should below to enhance the laser's efficiency, whereas the lifetime and thenonlinear loss q₀ should permit self-starting and stable cw modelocking.The saturable absorber's characteristics, as well as laser cavityparameters such as output coupling fraction, residual loss, and lifetimeof the gain medium, all play a role in the evolution of a laser fromstartup to modelocking.

To obtain rapid pulse shortening in a self-starting cw modelocked laserhaving a saturable absorber, the intensity on the saturable absorbershould be high and the absorber should have a nonlinear loss q₀ whosemagnitude is large. On the other hand, reducing the loss of thesaturable absorber causes the intracavity power to increase, which maylead to gain saturation. If the gain saturation does not dampen powerincreases caused by the large magnitude of the nonlinear loss q₀, thelaser will operate in a regime in which the laser Q-switches andmodelocks simultaneously (see H. A. Haus, “Parameter range for cwpassive mode locking,” IEEE, J. Quantum Electronics, QE-12, p. 169,1976). This is particularly true for a laser medium with a very longlifetime such as an erbium-doped fiber (τ˜ms). Thus, to avoidQ-switching, the magnitude of the nonlinear loss q₀ of the saturableabsorber must be limited, but not to the point where self-starting ofthe modelocking becomes difficult. The insertion loss and the nonlinearloss q₀ of a semiconductor saturable absorber can be controlled byselecting a material having the appropriate band gap and thickness.

The loss characteristics of a simple saturable absorber may be modifiedby the Fabry-Perot interference effect. Indeed, semiconductor saturableabsorbers tend to form a natural Fabry-Perot structure since asemiconductor's relatively high index of refraction (typically 2-4)results in a semiconductor-air interface from which ˜10-40% of theincident light may be reflected. A semiconductor saturable absorber mayhave one side that is high reflection coated (e.g., for maximumreflectivity), with this high reflector forming one end of a lasercavity. In this case, the fraction R_(F-P) of the intracavity power thatis reflected from the semiconductor saturable absorber is given byR _(F-P)=1−(1−R)(1−T)[1+RT+2(RT)^(1/2) cos(2δ)]⁻¹  (4)in which R is the front surface reflectivity of the saturable absorber(i.e., the reflectivity of the saturable absorber and any reflectioncoating thereon in the absence of reflection from the back side),δ=(2nd/λ)2π is the double pass phase change, d is the sample thickness,n is the index of refraction, and λ is the wavelength of interest. T isthe double pass transmission through the saturable absorber and is equalto exp(−2αd), with α being the absorption coefficient of the material.The corresponding absorption is then A=1−T=1−exp(−2αd). If multiplelayers with different indices of refraction and absorption coefficientsare used as part of the Fabry-Perot etalon, equation (4) must bemodified so that the double pass phase change and the absorption aresummed over all the layers.

The fraction of the laser cavity power incident on the Fabry-Perotstructure that is absorbed in the saturable absorber (F_(ABS)) is ingeneral not simply 1−T, but rather 1−R_(F-P). This is due to the factthat a Fabry-Perot structure acts as a resonating structure, in whichpower may circulate before reentering the rest of the laser cavity.

According to equation (4), R_(F-P) (the fraction of the intracavitystanding power reflected from a semiconductor saturable absorber) is asensitive function of the double pass phase change δ, which depends uponthe laser wavelength as well as the thickness and index of refraction ofthe saturable absorber. As illustrated in FIG. 1, for a given laserwavelength λ and index of refraction n, the reflectivity of aFabry-Perot device is a periodic function that depends upon thethickness d of the saturable absorber. If the thickness of theFabry-Perot device is chosen to be d=λm/2n, in which m is a positiveinteger, the double pass phase change is δ=2mπ, and the Fabry-Perotdevice is said to be at antiresonance. In this case,R_(F-P)=1−(1−R)(1−T)[1+(RT)^(1/2)]⁻².

In addition to wavelength and thickness, R_(F-P) can also be viewed as afunction of R. FIG. 2 considers how R_(F-P) varies as a function of Rand wavelength λ for a given saturable absorber thickness d. Inparticular, the higher R is, the more rapidly R_(F-P) varies. When R=0(i.e., when the surface of the saturable absorber that faces the gainmedium is anti-reflection coated), R_(F-P)=T and thus depends solelyupon the absorption of the saturable absorber. For a Fabry-Perotintracavity saturable absorber with a highly reflecting back surfacesuch as that considered here, it is often desirable to avoid the“etaloning” effect altogether by anti-reflection coating the surfacefacing the gain medium.

In general, however, R≠0, and by choosing d and R appropriately, theloss of a Fabry-Perot saturable absorber can be effectively controlled.If the thickness of the saturable absorber is chosen to be a multipleinteger of λ/2n, the device is said to be an anti-resonant Fabry-Perotsaturable absorber (A-FPSA) (see U. Keller et al., “Solid-state low-lossintracavity saturable absorber for Nd:YLF lasers: an antiresonantsemiconductor Fabry-Perot saturable absorber,” Opt. Lett., vol. 17, p.505, 1992 and U.S. Pat. No. 5,237,577 to Keller et al.) In an A-FPSA,the side of the device facing the gain medium usually includes a highreflector. In this configuration, most of the incident light isreflected from the gain-medium-facing surface and little goes into thesaturable absorber, thus reducing the light absorbed by the saturableabsorber. This low absorption design is appropriate for lasers withsmall output coupling and low single pass gain, such as solid statelasers. For example, if the laser has an output coupler of ˜4%, aninsertion loss of nearly 0.5% may be desirable, which is lower than whatis normally obtained from either a quantum well or bulk absorbersemiconductor. Low loss A-FPSA devices have been used successfully inmodelocked solid-state lasers (see, for example, U. Keller, D. A. B.Miller, G. D. Boyd, T. H. Chiu, J. F. Ferguson, and M. T. Asom,“Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers:an antiresonant semiconductor Fabry-Perot saturable absorber,” Opt.Lett., 17, 505, 1992).

Other low loss designs have been successfully used in modelockingarrangements. For example, a quantum well saturable absorber can beinserted into a Semiconductor Bragg Reflector (SBR) (see U.S. Pat. No.5,627,854 to Knox and also S. Tsuda, W. H. Knox, E. A. de Souza, W. Y.Jan, and J. E. Cunningham, “Low-loss intracavity AlAs/AlGaAs saturableBragg reflector for femto-second mode locking in solid state lasers,”Opt. Lett., vol. 20, p. 1406, 1995). In this arrangement, lightintensity decreases rapidly inside the SBR, and the insertion loss iscontrolled by precisely placing absorbing layers within the SBR.

Another means of manipulating the effective insertion and nonlinearlosses is through appropriate positioning of the absorber in a standingwave. In this design, an incident beam is reflected by a high or partialreflector to form an intracavity standing wave in which the intensityvaries between zero and twice the incident intensity. The insertion andnonlinear losses are controlled by appropriate positioning of absorbinglayers within the standing wave electric field. In U.S. Pat. No.5,701,327 to Cunningham et al., the quantum well absorption layers areinserted into a multiple half wavelength thick strain relief layer whichis then deposited on top of an SBR. Since the total thickness of thestrain relief layer is a multiple integer of half wavelengths a standingwave node is formed (where the intensity is minimum) at the surfacefacing the incident beam. This antiresonant design limits the amount oflight going into the strain relief layer and hence limits the amplitudeof the standing wave.

In another design (see U.S. Pat. No. 4,860,296 to Chemla et al.),nonlinear loss is maximized by placing thin absorbing layers (separatedby transparent spacers) at the antinodes of a standing wave to form a socalled grating saturable absorber. By placing the absorbing layers atthe antinodes, where the intensity is twice the average value, thenonlinear loss can be enhanced by up to a factor of 2 if the absorbinglayers are very thin compared to the transparent spacers.

All of these prior art designs involve saturable absorbers having lowinsertion loss. Accordingly, the magnitude of the nonlinear loss islimited, being maximized when the saturable absorber is completelybleached. For a high gain, high output fiber laser, however, themagnitude of the nonlinear loss is preferably large for modelocking tobe self-starting. On the other hand, the use of a highly nonlinearsaturable absorber may lead to persistent Q-switching. Thus, thereremains a need for saturable absorbers suitable for self-startingmodelocking of high gain, high output lasers such as fiber lasers.

SUMMARY OF THE INVENTION

The method of achieving self-starting cw mode-locking evolving fromQ-switched mode-locking (QSML) is disclosed. In contrast, themodelocking of most solid state lasers begins from cw noise.

The use of interactivity Resonant Fabry-Perot Absorbers (R-FPSA) forinducing self-starting mode-locking in a laser is also disclosed. Anoptical power limiter such as a two photon absorber (TPA), e.g., asemiconductor material, is optionally used in the laser cavity toinhibit Q-switching. The R-FPSA is designed such that the nonlinear lossexperienced by the saturable absorber is enhanced over the prior artA-FPSA configurations. The TPA power limiter provides effective damageprotection for the R-FPSA and self-adjusts the total nonlinear loss ofthe laser to be in the stable cw modelocking region.

The R-FPSA includes two reflectors having a spacing of roughly(2m+1)λ/4n. One reflector is preferably a maximum reflector that definesone end of the laser cavity (the “end reflector”), whereas the otherreflector is formed by a high or partial reflector that faces the gainmedium of the laser (the “inner reflector”). When the Fabry-Perot devicehas a thickness given by nd=(2m+1)λ/4, the double pass phase change isδ=(2m+1)π, and the Fabry-Perot structure is said to be at resonance. Inthis case, R_(F-P)=1−(1−R)(1−T)[1−(RT)^(1/2)]⁻² and is a minimum. Byoperating at resonance, the laser intensity absorbed by the saturableabsorber is enhanced. The absorbed intensity for the R-FPSA is given byI_(Abs)=(1−R_(F-P))I=(1−T)(1−R)/[1−(RT)^(1/2)]²I, as can be determinedfrom equation (4) with cos(2δ)=−1. This is to be compared with the casein which the front surface is anti-reflection coated (R=0) andI_(ABS)=(1−T)I. Thus, by operating the Fabry-Perot device at resonance,the intensity absorbed by the saturable absorber is increased by afactor of (1−R)/[1−(RT)^(1/2)]².

The effect of varying R on R_(F-P)(λ) for an R-FPSA is illustrated inFIG. 2. The spacing between adjacent minima is given byΔλ=λ_(m+1)−λ_(m)=λ_(m)λ_(m+1)/2nd and is preferably large for certainapplications such as ultrafast lasers, where broad bandwidth is needed.The inner reflector should have a reflectivity R sufficiently high toprovide a desired intensity on the saturable absorber. This reflectivityR, however, should not be so high that R_(F-P)(λ) is no longerrelatively flat over the gain profile. For example, if the innerreflector reflectivity R is too high, the bandwidth of R_(F-P)(λ) atresonance needed for modelocked laser pulses may be too limited. Forapplications in which the spot size on the saturable absorber can not bevaried (e.g., butt-coupling to a fiber or a waveguide), “tuning” theintensity on the absorber by selecting an appropriate R may bedesirable.

The resonant effect on the nonlinear loss and R_(F-P) as a function ofwavelength is explored in FIG. 3. This figure shows that the nonlinearloss experiences a significant enhancement when the Fabry-Perot deviceis designed to be at resonance. The negative nonlinear loss iscalculated as—q=R_(F-P)(R,T)−R_(F-P)(R,T₀). WhereT=exp(−2αd)=T₀exp(−2(δα)d)˜T₀(1−2(δα)d), with T=exp(−2α₀d)=50% and2(δα)d approximated as 0.2(1−R_(F-P)), proportional with the lightabsorbed in the sample. It can be seen that, the nonlinear loss atresonant (near 1540 nm) is 7 times larger than that at anti-resonant.

In one preferred embodiment, the gain medium is an erbium doped fiberhaving an upper state lifetime on the order of milliseconds (ms), andthe round trip cavity time is typically 10-100 nsec. By using an R-FPSAwith a large nonlinear loss, the fiber laser may operate in a QSMLregime rather than a cw modelocked regime. In this case, it may benecessary to suppress the intense Q-switched pulses, thereby driving thelaser below threshold. In a preferred embodiment of this invention, atwo photon absorber (TPA) is used for this purpose to complement theR-FPSA, so that the laser operates in a cw modelocked regime. The TPApreferably has little or no single photon absorption at the laserwavelength. Thus, two different types of absorbers, having differentnonlinear behavior, may be used in the same device to achieveself-starting, cw modelocked behavior.

The different intensity dependencies of a preferred saturable absorber(InGaAsP) and a preferred two photon absorber (InP) are illustrated inFIG. 4. The loss due to the two photon absorber increases strongly as afunction of intensity, whereas the loss due to the saturable absorberdecreases (saturates) with increasing intensity. The resultant“V-shaped” total loss of FIG. 4 has a minimum which is a favorableregime for cw modelocking.

The optical limiter (e.g., the TPA) preferably has a large two photonabsorption coefficient β₂, which is a function of the ratio of thematerial's band gap E_(g) and the photon energy,

$\frac{h}{2\pi}$(see, for example, E. W. Van Stryland, M. A. Woodall, H. Vanherzeele,and M. J. Soileau, “Energy band-gap dependence of two-photonabsorption,” Opt. Lett., 10, 490, 1985). FIG. 5 shows how the two photoncoefficient scales with this ratio, which is given by (Stryland et al.,supra):

${\left. {\beta_{2} = {{\kappa\left\lbrack {{\frac{h}{2\pi}/E_{g}} - 1} \right)}^{3/2}/\left( {\frac{h}{2\pi}/E_{g}} \right)^{5}}} \right\rbrack/n^{2}}E_{g}$Here κ is a nearly material independent parameter. For a given laserwavelength, the band gap E_(g) of the optical power limiter should belarger than the photon energy

$\frac{h}{2\pi},$so that maximum two photon absorption can be obtained withoutsignificant increase in the insertion loss. The band gap can be easilycontrolled by proper choice of the semiconductor material and/or itsdoping levels.

The TPA is effective at suppressing QSML regardless of its position inthe laser cavity. For example, the TPA may adjoin the saturableabsorber. Alternatively, the TPA and the saturable absorber may belocated on opposite sides of the gain medium, or several TPAs may beused to reduce the thickness of the Fabry-Perot device, thereby offeringgreater design flexibility (in accordance with equation (4)).

Suppression of Q-switched pulses by two photon absorbers has beenpreviously reported (see, for example, A. Hordvik, “Pulse stretchingutilizing two-photon-induced light absorption”, J. of QuantumElectronics, QE-6, 199 (1970) and V. A. Arsen'ev, I. N. Matveev, and N.D. Ustinov, “Nanosecond and microsecond pulse generation in solid-statelasers (review)”, Sov. J. Quantum Electron, vol. 7 (11), 1321 (1978)).Also, semiconductor-based two photon absorbers have been used as opticalpower limiters to protect damage sensitive optics (see, for example,U.S. Pat. No. 4,846,561 to Soileau et al.).

The band gap of a two photon absorber lies well above the photon energyat the laser wavelength, so that single photon absorption is low at lowintensities. At higher intensities, however, the production rate ofcarriers generated from the valance band to the conduction bandincreases. The absorption (1−T) from two photon effects is given by:A _(TPA)=β₂ Id _(TPA)/(1+β₂ Id _(TPA))  (6)where d_(TPA) is the thickness of the TPA material and β₂ is the TPAcoefficient. (See, for example, E. W. Van Stryland, H. Vanherzeele, M.A. Woodall, M. J. Soileau, A. Smirl, S. Guha, and T. F. Boggess, “Twophoton absorption, nonlinear refraction, and optical limiting insemiconductors”, Opt. Engin., vol. 24, 613, 1985).

A two photon absorber tends to limit the pulse shortening of highintensity pulses, since pulse peaks are more strongly attenuated thanthe wings. Thus, the conventional understanding of the two photonabsorption effect is that it degrades the performance of modelockedlasers (see, for example, A. T. Obeidat and W. H. Knox, “Effects oftwo-photon absorption in saturable Bragg reflectors in femtosecondsolid-state lasers”, OSA Technical Digest, 11, 130, Proceedings of CLEO'97). In the high gain fiber laser disclosed herein, however, Q-switchedmodelocking is the main impediment to cw modelocking. Thus, the twophoton absorber effectively suppresses QSML, thereby facilitating cwmodelocking, which is not significantly affected by the two photonabsorber. The result is that the intracavity use of one or more twophoton absorbers permits a wider range of saturable absorbers to beused.

The combination of the R-FPSA and the TPA optical limiter disclosedherein provides an ideal nonlinear device for self-starting modelocking,since the R-FPSA provides quick pulse shortening due to its largesaturable loss, and the optical limiter self adjusts the nonlinear lossto be within the cw modelocking stability region (FIG. 4). The TPA powerlimiter also provides effective damage protection for the saturableabsorber. The intensity on the saturable absorber can be optimized byvarying the spot size on the absorber, or by selecting R appropriately.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the thickness dependence of the reflectivity (R_(F-P)) of aFabry-Perot device for a laser wavelength of 1550 nm. The curve iscalculated using equation (4) with R=30% and T=50%.

FIG. 2 is a plot of R_(F-P) as a function of wavelength for varying R.The thickness of the device is 6.5 μm, yielding a bandwidth (full widthat half maximum) of ˜10 nm. The solid curve (R=0%) is indicative of thesemiconductor's band structure with the band gap at 1550 nm. The dottedcurves are calculated using equation (4) with the wavelength dependent Tbeing given by the R=0% case.

FIG. 3 shows R_(F-P) as a function of wavelength, calculated usingequation (4) with T=T₀=50% and R=30% The nonlinear loss is calculated as−q=R_(F-P)(R,T)−R_(F-P)(R,T₀). Where T=T₀(1−2(δα)d), with 2(δα)dapproximated as 0.2(1−R_(F-P)).

FIG. 4 illustrates the intensity dependence of loss due to saturableabsorber, loss due to a two photon absorber, and their sum. The loss dueto the saturable absorber is given by equation (1), with α being givenby equation (2).

FIG. 5 shows how the two photon coefficient varies as a function of

$\frac{h}{2\pi}/E_{g}$using equation (5).

FIG. 6 illustrates one embodiment in which a saturable absorber (forinducing modelocking) and a two photon absorber (to inhibit Q-switching)are adjacent each other.

FIG. 7 illustrates another embodiment, which is similar to the one ofFIG. 6, except that the saturable absorber and the two photon absorberare located at opposite ends of the laser cavity.

FIG. 8 illustrates how the negative nonlinear loss (measured using thepump-probe method with a fixed pump intensity) varies for variousInGaAsP saturable absorbers with different insertion loss. Data pointsrepresented by the open circles and solid dots were made with samplestaken from two different wafers. The square dots are AR-coated InGaAsPsamples for which the InP wafer was not removed.

FIG. 9 shows the fiber laser output as a function of time, illustratingthe evolution of Q-switched modelocking to cw modelocking. The signalstrength is normalized to cw ML signal.

FIG. 10 is a schematic of a monolithic R-FPSA device to be usedintracavity to generate cw modelocked pulses.

FIG. 11 is an embodiment for generating cw modelocked pulses in whichabsorbing layers are distributed at standing wave maximums. Thesine-like curve represents the intensity distribution of the standingwave within the absorbing material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In several preferred embodiments of this invention, self-starting cwmode-locking is obtained from Q-switched mode-locking. The cwmode-locking is induced by a near resonant Fabry-Perot saturableabsorber (R-FPSA). An optical limiter such as a two photon absorber ispreferably used in combination with the saturable absorber to selfadjust the nonlinear loss to be within the stability region for cwmode-locking.

An Integrated Saturable Absorber (R-FPSA)/Optical Power Limiter

A preferred embodiment of the invention is shown in FIG. 6, whichincludes a fiber gain medium 10 a such as a 1.5 m length of Er³⁺-dopedoptical amplifier fiber. The optical layout of this embodiment issimilar in some respects to those disclosed in Assignee's co-pendingU.S. application Ser. No. 09/040,252 filed Mar. 9, 1998 and entitledHIGH POWER, PASSIVELY MODELOCKED FIBER LASER, AND METHOD OFCONSTRUCTION, which is hereby incorporated by reference.

Pump light 14 a (preferably from a laser source, which is not shown inthe figures) with a wavelength near 980 nm is preferably directed intothe erbium fiber 10 a via a pump signal injector 18 a (connected to thefiber), such as a wavelength-division multiplexer (WDM), as shown in theexemplary cavity 20 a of FIG. 6. The pump light 14 a optically exciteserbium atoms in the fiber 10 a. A polarizing element 22 a such as apolarizing beam splitter cube or polarization beam splitter serves asthe output coupler for a short pulse modelocked fiber laser signal 16 a,16 a′ (in which 16 a and 16 a designate the leftward travelling andrightward travelling portions of the laser signal, respectively), andthe respective orientations of waveplate 26 a control the polarizationevolution inside the cavity 20 a and thus the level of output coupling.The laser signal 16 a, 16 a results from stimulated emission of excitederbium. Two Faraday rotators 30 a within the cavity 20 a (i.e.,proximate the ends of the cavity), which are preferably located onopposite sides of the fiber 10 a, compensate for polarization driftsinside the cavity.

Although the embodiments of the present invention are discussed hereinwith respect to the erbium laser described above, other fiber gain mediaand laser configurations can be used. For example, the fibers maycomprise other dopants (e.g., ytterbium, Thulium, Holmium, Neodymium,Praseodymium) and dopant concentrations, and different geometricalparameters can be used. Also, the laser configurations may includedouble clad fiber lasers, ring lasers, “FIG. 8” lasers, as well as otherconfigurations common in the art.

In the exemplary cavity 20 a, a saturable absorber 34 a, which may havean insertion loss of about 30-70%, is located at one end of the cavity.The saturable absorber 34 a is preferably InGaAsP attached to (i.e.,disposed directly on) a reflector such as a partially or preferablytotally reflecting minor 42 a, which together with a second highlyreflecting minor 38 a (or reflector) defines the extent of (i.e., theends of) the optical cavity 20 a. The saturable absorber 34 a inducesmodelocking of the laser signal 16 a, 16 a.

An optical power limiter 35 a (such as a two photon absorber) adjoiningan AR coating 36 a may optionally overlie the saturable absorber 34 a tosuppress Q-switching of the laser. The two photon absorber (TPA) 35 a ispreferably a 350 μm thick layer of InP. The reflectivity of the ARcoating 36 a is preferably small (i.e., <0.05%) to reduce opticalinterference effects. The mirror 42 a, saturable absorber 34 a, TPApower limiter 35 a, and the AR coating 36 a form an integral opticalstructure 37 a.

The laser beam from the fiber 10 a is collimated by a lens 47 a andrefocused by a lens 46 a onto the saturable absorber 34 a. The spot sizeon the saturable absorber may be adjusted by varying the position of thelens 46 a and 47 a or using lenses with different focal lengths. Otherfocusing lenses 48 a and 49 a in the cavity 20 a aid in better imagingthe laser signal 16 a, 16 a onto the fiber 10 a. The total intracavityfiber length within the cavity 20 a is preferably 2 m, and the signallaser preferably operates at a repetition rate of 50 MHz. The resultingmodelocked pulses are near bandwidth limited with a pulse width rangingfrom 300-600 fsec depending on the exact settings of the polarizationcontrolling elements and the degree of output coupling. For an inputpump power of 70 mW and optimized focusing on the saturable absorber 34a, the output coupling fraction can be varied between 50 and 80% and thelaser will still exhibit self-starting cw modelocking behavior. When theoutput coupling fraction is tuned below the lower limit (i.e. <40%),multiple pulsing (more than one pulse per round trip) or cw components,along with the mode-locked pulses, are typically observed, depending onthe insertion loss and the nonlinear loss experienced by the saturableabsorber 34 a.

A beam splitter 64 a (with a reflectivity of preferably 1-2%) may beinserted in front of the saturable absorber 34 a to monitor the power ofthe beams incident on (16 a, P_(Inc)) and reflected from (16 a′,P_(Ref)) the saturable absorber 34 a. The beamsplitter 64 a outcouplesfractions of the intracavity power designated by arrows 68 a and 70 a,which are proportional to P_(Inc) and P_(Ref), respectively.

Nonadjoining R-FPSA and Power Limiter

An alternative embodiment is shown in FIG. 7, in which numeralsjuxtaposed with the letter “b” are substantially similar to their “a”designated counterparts in FIG. 6. The embodiment of FIG. 7 differs fromthe one of FIG. 6 in that the TPA power limiter 35 b and its AR coating36 b no longer adjoin the saturable absorber 34 b, but rather the minor38 b on the other side of the cavity 20 b. The performance of the laserof FIG. 7 is nearly identical with that of FIG. 6.

Characterization of a Preferred Saturable Absorber: InGaAsP

A preferred saturable absorber is InGaAsP. A number of InGaAsP saturableabsorber elements were fabricated and then characterized in terms oflifetime, loss, and laser performance. The results of these tests aredescribed here.

1. Lifetime

The lifetime of the carriers in the InGaAsP saturable absorber 34 a, 34b was measured, using the pump-probe method, to be about 20 ns. However,the InGaAsP samples used in these embodiments were proton bombarded toreduce the carrier lifetime to ˜5 ps.

2. Nonlinear Loss

Two methods were used to determine the in situ, operating nonlinear lossq_(op) experienced by the saturable absorber 34 b, i.e., the differencebetween the loss of the saturable absorber under cw modelocked operationand at lasing threshold:q _(op) =l(I at modelocking)−l(I at lasing threshold)  (7)The most straightforward approach is to monitor the outcoupled powers 64b and 68 b to determine the ratio between P_(Ref), the power reflectedfrom the saturable absorber, and P_(Inc), the power incident on thesaturable absorber, at both lasing threshold (relatively low intracavitypower) and under modelocked conditions. P_(Ref)/P_(Inc) is simplyR_(F-P), and ΔR_(F-P)=−ΔF_(ABS)=−q_(op), in which F_(ABS) is thefraction of the power incident on the saturable absorber 34 b that isabsorbed. Thus, measuring the change in P_(Ref)/P_(Inc) between thelasing threshold case and modelocked operation directly gives thenonlinear loss q_(op) experienced by the saturable absorber 34 b.

In the second approach, the saturable absorber 34 b (and the minor 42 bthat underlies it) are first tested in an extracavity configurationusing the pump probe method to determine the nonlinear loss q of thesaturable absorber as a function of incident beam intensity, i.e., acalibration curve is constructed. In this technique, the nonlinear lossq is the change in R_(F-P) of the probe beam (off of the saturableabsorber 34 b) in the limit of zero time delay from the pump beam. Next,the saturable absorber 34 b to be tested (and its mirror 42 b) areintroduced into a laser cavity like that shown in FIG. 7. Theintracavity power (P_(Inc), 16 b) is monitored by measuring the poweroutcoupled in the direction indicated by the arrow 68 b. Once theintensity on the saturable absorber 34 b is in hand, the nonlinear lossq can be determined from the calibration curve. Although the results ofthese two methods are consistent with each other, the second methodgives more accurate results.

3. Laser Performance

Laser performance was studied as a function of saturable absorber lossto establish a working range over which the laser displays self-startingcw modelocking behavior. For a fixed intensity, the nonlinear loss of asaturable absorber may vary significantly as is shown in FIG. 8, forwhich the data were measured using the pump-probe method for a givenpump beam intensity. The nonlinear loss of the saturable absorber 34 bunder modelocking conditions was determined for a number of differentsaturable absorbers using the methods described in the above section.

Values of |q_(op)| of approximately 15% resulted in satisfactory cwmodelocking performance with respect to self-starting behavior, andperturbation stability. For still lower values of |q_(op)|,self-starting becomes difficult. The performance of saturable absorbershaving low |q_(op)| under low intensity conditions can usually beimproved by increasing the intensity on the saturable absorber until|q_(op)| is increased up to, for example, the 15% level. As discussedpreviously, the intensity on the saturable absorber can be increasedthrough either tighter focusing or by decreasing the output couplingfraction. For saturable absorbers providing very high nonlinear loss(|q_(op)|>20%) due to, for example, the choice of band gap, onlyQ-switched mode-locking as opposed to cw modelocking was observed.

The maximum output coupling fraction consistent with self-startingmodelocking behavior decreases with decreasing insertion loss for agiven spot size, or alternatively, increasing spot size for a giveninsertion loss. For all of the saturable absorbers studied, the bestperformance results were obtained when the spot size on the saturableabsorber was as small as possible without damaging it.

Evolution of cw Modelocking from Q-Switched Modelocking (QSML)

A fast detector and a digital oscilloscope were used to record theevolution of laser pulses from the embodiment of FIG. 6. These resultsare indicated in FIG. 9 and are a temporal illustration of howQ-switched modelocking evolves into cw modelocking. Similar results wereobtained with the configuration of FIG. 7. The peak power of most of theQ-switched pulses can be a factor of 30 greater than that of the cwmodelocked pulses. However, a couple of relatively lower powerQ-switching pulses usually precede the transition to cw modelocking,with cw modelocking evolving out of the tail of one of the QSML pulses.

The pulse width of the Q-switched mode-locking pulses was measured withan auto-correlator to be less than 3 ps. To do this, the laser isdeliberately set to the QSML regime by tuning the output coupling.

Exemplary Embodiments Including a TPA

The optical limiters of this invention are preferably made fromsemiconductors with a large two photon absorption coefficient. Althoughthe embodiments and results herein are described primarily with respectto InP/InGaAsP materials, other semiconductor materials may be employed.For applications near 1.55 μm, InP, InGaAsP, GaAs, AlGaAs may besuitable for use as the TPA power limiter. At shorter wavelengths, ZnS,CdSe, CdS, and CdTe can be used as TPAs. Sophisticated materialengineering techniques, such as doping (which creates intermediatestates), low temperature growth, quantum confinement, and latticemismatch may also be employed to enhance the TPA effect, therebyproducing a strong optical limiter.

One preferred two photon absorber is InP, which may also serve as thesubstrate for InGaAsP, which is a preferred saturable absorber. As partof these studies, the two photon absorption loss of a 350 μm thicksample of InP was measured in an extracavity test setup (not shown), inwhich one side of the sample was AR-coated and the other side was coatedwith gold. The results were found to be in excellent agreement with thetheoretical values presented in FIG. 4. Also, the two photon absorptioncoefficient was determined to be 18 cm/GW, which is in good agreementwith theoretical values. The nonlinear loss of the TPA sample wasdetermined to be 50% at an intensity of approximately 0.2 GW/cm². Undervery tight focusing conditions (spot size approx. 20 μm²), opticaldamage was observed at the AR coated surface for intensities on theorder of 10 GW/cm², which is much higher than the peak intensity of theQSML pulses (1 GW/cm²) of the fiber laser disclosed herein.

The effect of the TPA power limiter was studied in differentexperimental configurations and the cw mode-locked laser performance wasthen compared with that when the TPA is absent. In one embodiment, asshown in FIG. 6, a 350 μm layer of InP, the substrate, was left on thetop of the ˜0.75 μm InGaAsP absorber layer. The InP side surface isAR-coated and the InGaAsP side surface is HR coated. The insertion lossfrom the saturable absorber layer 34 a for these tests was about 45%These results were then compared with a modification to this embodiment,in which the InP layer 35 a and its AR coating 36 a were not used. Thelaser provided nearly identical performance regarding the output power,wavelength, self-starting and stability against perturbation, comparedto when the InP layer 35 a was removed. For the configurationrepresented by FIG. 6, the spot size on the absorber 34 a was variedfrom between 12 and 5 microns, but no damage to the saturable absorberwas observed even for the highest possible pump power (70 mW).

Another set of comparisons was performed using the embodiment of FIG. 7.In FIG. 7, the InP two photon absorption layer 35 b and the saturableabsorber 34 b are located on opposite ends of the cavity 20 b. In thisconfiguration, the performance of the laser is analyzed both with theInP wafer inside the cavity and InP wafer removed. When the InP layer 35b is removed from the cavity, the saturable absorber 34 b was easilydamaged during QSML when the 980 nm pump diode power is more than 60 mWat a spot size of ˜12 μm (diameter). With the InP layer 35 b and 36 b inthe cavity, no damage on the InGaAsP saturable absorber 34 b wasobserved for the highest possible pump level (˜70 mW) and with a spotsize as small as ˜8 μm. For the embodiment of FIG. 7, the peakintensities of the QSML pulses was also monitored during the startupprocess of the mode-locking (similar to that in FIG. 9). We found thatwith the InP wafer in the cavity, the peak intensity of the QSML pulsesis reduced to ⅕ of that when the InP is absent. In general, the use ofan optical power limiter such as a two photon absorber reduces the peakpower of optical pulses, hence, effectively protecting the saturableabsorber from damage.

Since an optical limiter such as a two photon absorber does introducesome additional loss, a decrease in the cw modelocked output power canalso be expected. Indeed, the outcoupled cw modelocked power from theconfiguration of FIG. 7 was about 5% less than that of the same laserbut with the InP (35 b and 36 b) removed from the cavity for a pulseenergy of approximately 0.2 nJ.

As the above discussion suggests, the use of a TPA optical power limiterpermits greater latitude in designing and selecting a saturable absorberand reduces the possibility of damage to the saturable absorber. Becausea TPA offers protection against optically or thermally induced damage,there is greater latitude in choosing the spot size on the saturableabsorber, i.e., in varying the nonlinear loss. Further, the nonlinearloss of a TPA tends to force the fiber laser to operate in a cwmodelocking regime.

Alternative Modelocking/Power Limiting Embodiments

1. Distributed Bragg Reflector as the High Reflector

A monolithic Resonant Fabry-Perot Saturable Absorber (R-FPSA) structure100 that can be substituted for the optical structure 37 a of FIG. 6 isillustrated in FIG. 10. This device preferably has a total thickness of6.5 μm to provide a relatively wide bandwidth on the order of 50 nm. Inthis device, a quarter-wave distributed Bragg reflector (DBR) 104 servesas the high reflector and preferably overlies (and may be grown on) asemiconductor substrate 108. A power limiter 35 a′ such as a TPA mayadjoin the DPR 104, which in turn is preferably adjoined by a saturableabsorber 34 a′. A reflecting layer 112 preferably overlies the saturableabsorber 34 a′ and acts as a partially reflecting surface. Thereflectivity of the reflecting layer 112 may be chosen to give theoperator greater freedom in choosing the tightness of the intracavityfocusing. The saturable absorber 34 a′ and the power limiter 35 a′function much like their unprimed counterparts of FIG. 6.

One design consideration with respect to the Fabry-Perot devicesdisclosed herein is that their free spectral range Δλ should be largecompared to the bandwidth of the laser pulses. Thus, the total thicknessof the absorbing layer plus any two photon absorbing layer is preferablylimited to t=λ²/2nΔλ. For example, if the desired Δλ is 50 nm at 1.5 μm,the thickness of the resonant cavity should be less than 6.5 μm for anindex of refraction of ˜3.5. On the other hand, for a given Fabry-Perotcavity thickness, the reflectivity R should be selected so that R_(F-P)is preferably relatively flat over the laser gain profile, so that thelasing wavelength does not shift too far away from the gain peak.

The positions of the saturable absorber 34 a′ and the TPA layer 35 a′may be exchanged to correspond with the ordering of the layers shown inFIG. 6. Further, in this and the other embodiments disclosed herein, thesaturable absorber and the TPA may be positioned at any point in theoptical device, and in fact, the TPA may be distributed throughout thedevice.

2. A Distributed Saturable Absorber and TPA

In the embodiment of FIG. 11, saturable absorber material 120 andoptical limiting material 126 (e.g., a two photon absorber) aredistributed throughout part of a laser cavity 20 c, which is otherwisesimilar to its counterpart in FIG. 6. In this embodiment, the absorbingmaterial 120 is preferably thin layers located at antinodal points of aFabry-Perot cavity standing wave 130, with the TPA material 126 beingused as a spacer between the thin absorbing layers. Since the fractionof light entering the device is a function of the front surfacereflection, the amplitude of the standing wave formed inside the devicedepends on the front surface reflector 42 c. In this situation, thestanding wave is formed between the light propagating toward the backhigh reflector and that reflected by the back high reflector. Theintensity on the absorbing layers 120, hence, can be controlled like theother R-FPSA devices disclosed herein.

3. Tunable Band Gaps

The saturable absorbers may be tuned to a limited extent by alteringtheir effective band gap. This can done by changing the carrierinjection rate through biasing, e.g., applying electrical leads to thesaturable absorber. A similar technique has been used with verticalcavity surface emitting lasers (VCSELs). (See, for example, J. A.Hudgings, S. F. Lim, G. S. Li, W. Yuen, K. Y. Lau, and C. J.Chang-Hasinain, “Frequency tuning of self-pulsating in VCSEL with avoltage-controlled saturable absorber”, OSA Technical digest 12, 10, OFC'98). Electrical signal from the electrical leads can also be used tomonitor the photocurrent generated from optical pulses. Such signal canalso be used for synchronization purposes.

4. Other Embodiments

In addition to the amplitude response of two photon absorbers (TPAs)discussed so far, TPAs also have a phase response, where the phaseresponse arises from nonlinear changes in the refractive index of theTPAs. Semiconductor two photon absorbers are in general affected by theresponse of bound as well as free carriers. The response time of boundand free carriers can be quite different, ranging from about 300 fs forbound carriers to a range of ˜1 ps-30 ns for free carriers. Theamplitude response of semiconductor TPAs operating at laser frequenciesabove half band gap is mainly due to bound carriers, whereas the phaseresponse is affected by bound as well as free carriers. Thus, the phaseresponse of TPAs can have a very much longer lifetime compared to theamplitude response.

The nonlinear refractive index changes lead to self-focusing ordefocusing; in a semiconductor operated well above half band gap,self-defocusing is typical. Self-defocusing as well as self-focusinglead to intensity-dependent changes of the divergence of the opticalbeams inside a laser cavity. The transmission through an aperturelocated somewhere inside the laser cavity can thus be adjusted todecrease as a function of intensity, and an effective optical limitingeffect can hence also be obtained by the phase-response of the TPA. Notethat in a cavity comprising a single mode fiber, the fiber itselfprovides such an aperture, and a cavity that comprisesphase-response-induced optical limiting looks similar to either FIG. 6or 7 and is not separately shown.

Self-defocusing can also lead to a further optical limiting mechanism.This may be explained as follows. Let us assume an optical element wherethe TPA is integrated with the SA and a high reflector (HR), where theTPA is located in front of the SA and the adjacent HR, and let the wholestructure be positioned at one end of a Fabry-Perot cavity as shown inFIG. 6. The intensity of the optical beam impinging onto the SA absorbercan then be limited by self-defocusing in the TPA, which leads to anincrease in the beam diameter on the SA with an increase in intensity.

Moreover, since the TPA-induced index changes can be relativelylong-lived and comparable to the cavity round-trip time, the amount ofself-defocusing can accumulate from pulse to pulse leading to a furtherenhancement of the optical limiting effect in a Q-switched mode-lockedlaser.

It should be understood that the scope of the present invention is notlimited by the illustrations or the foregoing description thereof, butrather by the appended claims, and certain variations and modificationsof this invention will suggest themselves to one of ordinary skill inthe art.

1. A mode locked fiber laser comprising: an optical cavity comprising anoptical limiter (OL) and a resonant Fabry-Perot saturable absorber(R-FPSA), wherein each of the OL and the R-FPSA comprise non-linearabsorption characteristics; and a gain medium comprising a doped opticalfiber, the gain medium disposed within the optical cavity; wherein themode locked fiber laser is configured to operate in a regime wherein thecombined total loss of the R-FPSA and the OL as a function of intensityis near a local minimum, such that the R-FPSA and the OL, incombination, evolve cw modelocking from Q-switched modelocking.
 2. Themode locked fiber laser according to claim 1, wherein the OL comprises asemiconductor material.
 3. The mode locked fiber laser according toclaim 1, wherein the OL comprises a two photon absorber.
 4. The modelocked fiber laser according to claim 3, wherein the two photon absorberhas relatively little single photon absorption at the laser wavelength.5. The mode locked fiber laser according to claim 3, wherein the twophoton absorber has a band gap that is larger than the photon energy atthe laser wavelength.
 6. The mode locked fiber laser according to claim1, wherein the R-FPSA induces cw mode locked pulses and the OLpreferentially suppresses Q-switched pulses.
 7. The mode locked fiberlaser according to claim 1, further comprising a polarization driftcompensator disposed in the optical cavity.
 8. The mode locked fiberlaser according to claim 1, wherein the R-FPSA comprises a distributedBragg reflector and a saturable absorber.
 9. The mode locked fiber laseraccording to claim 8, wherein the OL and the saturable absorber aredisposed between the distributed Bragg reflector and a partiallyreflecting layer.
 10. The mode locked fiber laser according to claim 9,wherein the OL adjoins the distributed Bragg reflector.
 11. The modelocked fiber laser according to claim 9, wherein the reflectivity of thepartially reflecting layer is selected to provide a desired amount ofintracavity focusing.
 12. The mode locked fiber laser according to claim1, wherein the R-FPSA comprises InGaAsP.
 13. The mode locked fiber laseraccording to claim 1, wherein the R-FPSA has a free spectral range thatis large compared to a bandwidth of the mode-locked laser pulses. 14.The mode locked fiber laser according to claim 1, wherein the opticalcavity comprises a first reflector and a second reflector, the firstreflector comprising the R-FPSA, and the OL adjoining the secondreflector.
 15. The mode locked fiber laser according to claim 1, whereinthe mode locked fiber laser is configured to operate in a regime suchthat the nonlinear operating loss of the R-FPSA is up to about 15%.